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Oxazole is the parent compound for a vast class of heterocyclic
aromatic In chemistry, aromaticity is a chemical property of cyclic (ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to sat ...
organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon- hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s. These are azoles with an oxygen and a nitrogen separated by one carbon. Oxazoles are aromatic compounds but less so than the thiazoles. Oxazole is a weak base; its
conjugate acid A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...
has a p''K''a of 0.8, compared to 7 for
imidazole Imidazole (ImH) is an organic compound with the formula C3N2H4. It is a white or colourless solid that is soluble in water, producing a mildly alkaline solution. In chemistry, it is an aromatic heterocycle, classified as a diazole, and has non ...
.


Preparation

Classical oxazole synthetic methods in
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...
are * the Robinson–Gabriel synthesis by dehydration of 2-acylaminoketones * the Fischer oxazole synthesis from cyanohydrins and aldehydes * the Bredereck reaction with α-
haloketone In organic chemistry, an α-haloketone is a functional group consisting of a ketone group or more generally a carbonyl group with an α- halogen substituent. α-haloketones are alkylating agents. Prominent α-haloketones include phenacyl bromid ...
s and formamide * the Van Leusen reaction with aldehydes and TosMIC Other methods: * Oxazolines can also be obtained from cycloisomerization of certain propargyl
amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
s. In one study oxazoles were prepared via a one-pot synthesis consisting of the condensation of propargyl amine and
benzoyl chloride Benzoyl chloride, also known as benzenecarbonyl chloride, is an organochlorine compound with the formula . It is a colourless, fuming liquid with an irritating odour, and consists of a benzene ring () with an acyl chloride () substituent. It i ...
to the
amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
, followed by a Sonogashira coupling of the terminal alkyne end with another equivalent of benzoylchloride, and concluding with ''p''-toluenesulfonic acid catalyzed
cycloisomerization Cycloisomerization is any isomerization in which the cyclic compound, cyclic isomer of the substrate is produced in the reaction coordinate. The greatest advantage of cycloisomerization reactions is its atom economy, atom economical nature, by desig ...
: : *In one reported oxazole synthesis the reactants are a nitro-substituted
benzoyl chloride Benzoyl chloride, also known as benzenecarbonyl chloride, is an organochlorine compound with the formula . It is a colourless, fuming liquid with an irritating odour, and consists of a benzene ring () with an acyl chloride () substituent. It i ...
and an isonitrile: :


Biosynthesis

In biomolecules, oxazoles result from the cyclization and oxidation of serine or threonine nonribosomal peptides: : Oxazoles are not as abundant in biomolecules as the related
thiazole Thiazole, or 1,3-thiazole, is a heterocyclic compound that contains both sulfur and nitrogen. The term 'thiazole' also refers to a large family of derivatives. Thiazole itself is a pale yellow liquid with a pyridine-like odor and the molecular fo ...
s with oxygen replaced by a sulfur atom.


Reactions

With a pKa of 0.8 for the conjugate acid, oxazoles are far less basic than imidazoles (pKa = 7).Thomas L. Gilchrist "Heterocyclic Chemistry" 3rd ed. Addison Wesley: Essex, England, 1997. 414 pp. . * Deprotonation of oxazoles at C2 is often accompanied by ring-opening to the isonitrile. *
Electrophilic aromatic substitution Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic ni ...
takes place at C5 requiring
activating group In electrophilic aromatic substitution reactions, existing substituent groups on the aromatic ring influence the overall reaction rate or have a directing effect on positional isomer of the products that are formed. An electron donating group ( ...
s. * Nucleophilic aromatic substitution takes place with leaving groups at C2. * Diels–Alder reactions with oxazole dienes can be followed by loss of oxygen to form
pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakl ...
s. * The Cornforth rearrangement of 4-acyloxazoles is a thermal rearrangement reaction with the organic acyl residue and the C5 substituent changing positions. * Various
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
reactions. One study"Ceric Ammonium Nitrate Promoted Oxidation of Oxazoles", David A. Evans, Pavel Nagorny, and Risheng Xu. ''
Org. Lett. ''Organic Letters'' is a biweekly peer-reviewed scientific journal covering research in organic chemistry. It was established in 1999 and is published by the American Chemical Society. In 2014, the journal moved to a hybrid open access publishin ...
''; 2006; 8(24) pp 5669 - 5671; (Letter)
reports on the oxidation of 4,5-diphenyloxazole with 3 equivalents of CAN to the corresponding
imide In organic chemistry, an imide is a functional group consisting of two acyl groups bound to nitrogen. The compounds are structurally related to acid anhydrides, although imides are more resistant to hydrolysis. In terms of commercial applicati ...
and benzoic acid: : : In the balanced half-reaction three equivalents of water are consumed for each equivalent of oxazoline, generating 4 protons and 4 electrons (the latter derived from CeIV).


See also

*
Isoxazole Isoxazole is an electron-rich azole with an oxygen atom next to the nitrogen. It is also the class of compounds containing this ring. Isoxazolyl is the univalent radical derived from isoxazole. Occurrence Isoxazole rings are found in some natur ...
, an analog with the
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas ...
in position 2. *
Imidazole Imidazole (ImH) is an organic compound with the formula C3N2H4. It is a white or colourless solid that is soluble in water, producing a mildly alkaline solution. In chemistry, it is an aromatic heterocycle, classified as a diazole, and has non ...
, an analog with the
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...
replaced by a nitrogen. *
Thiazole Thiazole, or 1,3-thiazole, is a heterocyclic compound that contains both sulfur and nitrogen. The term 'thiazole' also refers to a large family of derivatives. Thiazole itself is a pale yellow liquid with a pyridine-like odor and the molecular fo ...
, an analog with the oxygen replaced by a sulfur. * Benzoxazole, where the oxazole is fused to another aromatic ring. * Pyrrole, an analog without the oxygen atom. * Furan, an analog without the nitrogen atom. * Oxazoline, which has one double bond reduced. * Oxazolidine, which has both double bonds reduced. *
Oxadiazoles Oxadiazoles are a class of heterocyclic aromatic chemical compound of the azole family; with the molecular formula C2H2N2O. There are four isomers of oxadiazole: File:1,2,3-oxadiazole.svg, 1,2,3-oxadiazole File:1,2,4-oxadiazole.svg, 1,2,4-oxad ...
with two nitrogens instead of one (e.g. furazan). * Oxazolone, an analog with a carbonyl group


References

{{reflist Simple aromatic rings